Last year we reported the development of a very sensitive radioimmunoassay for cyclic 3'5'cytidine monophosphate. Utilizing this method in combination with ion-exchange chromatography on Dowex 1 columns we have clearly demonstrated that cyclic CMP does not exist in various tissues or biologial fluids. Further we had shown that the rat pancreas which appeared to contain some apparent cyclic CMP is probably due to cyclic 2'3'-CMP which is formed from RNA. So the existence of cyclic CMP or its role in cell proliferation is highly questionable.